For amplifying micro-current in a light receiving element such as a photodiode or the like, the current is generally converted to voltage using an amplifier which has less input bias current and small input off-set voltage and drift voltage. When amplifying the micro current, a circuit system called a transimpedance amplifier is used. In the circuit system, the photodiode is directly inserted into amplifier input-portion.
A conventional light receiving circuit includes an inversion amplifier and a light receiving element, the inversion amplifier having a feedback resistor being connected between an input terminal and an output terminal, the light receiving element being connected with an input terminal of the inversion amplifier.
Further, the conventional light receiving circuit includes a comparator which compares an output of the inversion amplifier and a threshold voltage. Further, the comparator outputs a high-level voltage or a low-level voltage corresponding to the comparison result.
When a pulse optical signal is entered into the light receiving element, a current is provided from the light receiving element into the inversion amplifier corresponding to the optical signal. The current is transformed to voltage in the inversion amplifier and a pulse voltage is output corresponding to the optical signal.
The comparator compares between the pulse voltage and the threshold voltage to detect the pulse voltage above the threshold voltage as optical signal.
However, with increasing optical signal strength entered into the light receiving element, a problem generating a distortion in the pulse voltage wave-form output from the comparator so that the light receiving circuit may malfunction.
Therefore, when a gain of an inversion amplifier is set at a low level, detecting the optical signal becomes difficult with decreasing incident optical signal strength entered into the light receiving element, so that a dynamic range of the light receiving circuit is narrowed.
On the other hand, a light receiving circuit having an automatic-gain control circuit is disclosed, for example, in Japanese Patent Publication (Kokai) No. 2003-258580.
The light receiving circuit disclosed in Japanese Patent Publication (Kokai) No. 2003-258580 includes a pre-amplifier converting a current signal to a voltage signal so as to output, a lower-frequency-region passing filter flattening the voltage signal output from the pre-amplifier so as to output the lower-frequency-region filtration signal. The light receiving circuit performs automatic-gain control by changing a feedback resistance value of the pre-amplifier using the signal through the lower-frequency-region passing filter.
In the other hand, the light receiving circuit disclosed in Japanese Patent Publication (Kokai) No. 2003-258580 performs automatic-gain control on the basis of average strength in the optical signal, however, does not disclose with respect to a problem generated by an overshoot optical signal instantaneously input.